kas: split into unique files and add testimage

Refactor the existing kas configuration by splitting it into a shared
base.yml and a set of per-machine include files. This allows for easier
usage and potential CI use cases.

Also, change the readme to reflect the new changes.

Signed-off-by: Jon Mason <jon.mason@arm.com>

kas/raspberrypi3-64.yml

@@ -0,0 +1,14 @@
+# yaml-language-server: $schema=https://raw.githubusercontent.com/siemens/kas/master/kas/schema-kas.json
+
+header:
+  version: 14
+  includes:
+    - kas/base.yml
+
+local_conf_header:
+  license: |
+    # Uncomment next line to allow the license
+    # See: linux-firmware-rpidistro in docs/ipcompliance.md
+    LICENSE_FLAGS_ACCEPTED = "synaptics-killswitch"
+
+machine: raspberrypi3-64